The present invention relates to a power module which uses a power semiconductor device such as an insulated gate bipolar transistor (IGBT) for a switching device. More specifically, the present invention relates to a terminal wiring structure of the power module.
A conventional power module disclosed in the Japanese Patent Publication (KOKAI) No. H07-321285 includes a circuit block, a metal base, an integral resin casing, a casing cover and a sealing resin. The circuit block includes an insulative substrate such as a ceramic substrate on which circuit parts and components including a power semiconductor device such as an IGBT are mounted. The metal base is laminated on the back surface of the circuit block. The integral resin casing surrounds the circuit block and is bonded with adhesives to the periphery of the metal base. The integral resin casing is integrally united with lead-out terminals such as main terminals and auxiliary terminals. The main terminals include a collector terminal and an emitter terminal. The auxiliary terminals include auxiliary signal terminals for the emitter and gate. The integral resin casing is formed, by insert-molding, integrally with the terminal frames of the main and auxiliary terminals inserted thereto. Inner leads extend from the terminal frames and are wired inside the resin casing. The end sections of the inner leads are soldered to a circuit pattern on the substrate of the circuit block.
In another conventional power module which includes an integral resin casing formed, by insert-molding, integrally with lead-out terminals inserted thereto, auxiliary terminals are gathered in a peripheral portion of the resin casing. The terminal frames of the auxiliary terminals are buried in the wall of the resin casing and extended to the vicinity of soldering locations of the circuit block so as no to cross the wiring paths of the bonding wires of the circuit block inside the resin casing so that the terminal frames do not interact with the bonding wires. Inner leads extend inwardly from the terminal frames to the soldering locations, and the end sections of the inner leads are soldered to the substrate of the circuit block.
FIGS. 5(a) and 5(b) show the external appearance of the conventional power module including two IGBTs connected in series. FIG. 6 is an equivalent circuit of the power module of FIGS. 5(a) and 5(b). Referring now to FIGS. 5(a) and 5(b), the reference numeral 1 designates an integral resin casing integrally united with a lead-out terminal; numeral 2 is a casing cover; numeral 3 is a metal base on the bottom of the resin casing; numeral 4 is a main circuit terminal; and numeral 5 is an auxiliary signal terminal. The main circuit terminals 4 are led outside through the casing cover 2. The auxiliary terminals 5 are gathered on a peripheral portion of the resin casing 1. Each terminal is soldered to the substrate of a circuit block (not shown) mounted inside the resin casing such that the equivalent circuit of FIG. 6 is formed.
Referring now to FIG. 6, the symbol Tr1 designates an IGBT; Tr2 is another IGBT; and D is a free-wheeling diode connected parallel to the IGBT. The IGBTs Tr1 and Tr2 are mounted on the circuit block. The main circuit terminals 4 and the auxiliary terminals 5 are further designated by the terminal symbols for the collector, emitter and gate such as C1, E2, C2E1, G1, E1, G2 and E2.
The conventional internal wiring structure described above has the following problems in manufacturing and reliability.
The integral resin casing is formed through the steps of inserting the terminal frames for the main terminals and auxiliary terminals in the predetermined positions of a molding die, closing the molding die and supplying a molding resin through a gate of the molding die. The molding pressure which reaches 300 to 400 kg/cm.sup.2 may cause the inserted terminal frames to move from the predetermined positions or may deform the terminal frames. Especially, the auxiliary terminal frame in the form of a strip and buried in the wall of the resin casing is easily deformed under the molding pressure.
The middle portion of the auxiliary terminal frame inserted in the wall of the resin casing may be pushed to the outside of the casing wall and exposed. Or, the terminal frames for the terminals with opposite polarities may contact with each other in the resin layer of the casing. The exposed terminal frame or the short-circuited terminal frames are molding defects. The deformation of the terminal frame further causes random displacement of the end sections of the inner leads extending inside the casing from the predetermined soldering positions. The displaced inner leads are hazardous for soldering and cause problems in assembling the power module.
In addition, since the materials for the resin casing, metal base, ceramic insulative substrate, terminal frames and semiconductor devices exhibit different thermal expansion coefficients, thermal stress is exerted to the bonding portions of the constituent parts and components by the heat cycles due to the current carrying operations of the semiconductor devices. Especially, when the thermal stress is exerted repeatedly to the soldered portions of the end section of the inner lead and the wiring pattern of the circuit block, cracks are formed in the solder layer. As the cracks grow, the soldered faces are completely separated from each other to thereby causes a critical fault such that the semiconductor device stops operating.
In view of the foregoing, it is an object of the invention to provide a stable and reliable power module which obviates the foregoing problems.
It is another object of the invention to provide a power module which has an improved terminal wiring structure.